The hemostatic effect of platelets has been well established, but the possible role of red cells in hemostasis has not yet been well studied. An evaluation of the hemostatic effect of packed red cell transfusion in patients with chronic anemia was the purpose of this study. In a prospective study, bleeding time (BT), activated partial thromboplastin time (APTT), and prothrombin time (PT) were measured before and after the transfusion of allogeneic packed red cells in 42 patients with chronic anemia. The results were compared and analyzed. APTT and BT decreased significantly after transfusion, by a mean of 1.3 seconds (p = 0.01) and 2.6 minutes (p < 0.01), respectively. PT did not change significantly after transfusion (p = 0.65). Factors studied (patient's age, sex, and renal function measurements; pretransfusion and posttransfusion hemoglobin levels, platelet counts, and PTs; change in platelet count [delta platelet count] and PT [delta PT] after transfusion) did not independently affect the change in BT (delta BT) or in APTT (delta APTT). The delta BT was not affected by the pretransfusion or posttransfusion levels of APTT or by the delta APTT. The delta APTT was not affected by the pretransfusion or posttransfusion levels of BT or by the delta BT. Diagnosis of malignant or benign diseases was found to affect delta APTT, but not delta BT. Patients with pretransfusion hemoglobin < or = 60 g per L had a 4.07 times greater chance of posttranfusion increase in BT than the patients with hemoglobin > 60 g per L (p = 0.04). Red cell transfusion might decrease the APTT and BT in some anemic patients, though the actual cause of the decrease was not determined in the present study.